Specialty pigments have been developed for use in security applications, such as anti-counterfeiting devices printed on banknotes, packaging of high-value items, seals for containers, and even for direct application to commercial items. For example, the U.S. twenty-dollar Federal Reserve Note currently uses optically variable ink The number “20” printed in the lower-right corner of the face of the note changes color as the viewing angle changes. This is an overt anti-counterfeiting device. The color-shifting effect is not reproducible by ordinary color photocopiers, and someone receiving a note can observe whether it has the color-shifting security feature to determine the note's authenticity.
Other high-value documents and objects use similar measures. For example, iridescent pigments or diffractive pigments are used in paints and inks that are applied directly to an article, such as a stock certificate, passport, original product packaging, or to seals that are applied to an article. Security features that are more difficult to counterfeit are desirable as counterfeiters continue to become more sophisticated.
One anti-counterfeiting approach uses microscopic symbols on multi-layer color-shifting pigment flakes. The symbols are formed on at least one of the layers of the multi-layer color-shifting pigment flakes by a local change of an optical property(s), such as reflectivity. The multi-layer color-shifting pigment flakes may have an all-dielectric design, or a metal-dielectric design. The symbols may be stamped or embossed or etched in the pigments by mechanical means or formed by laser means.
The microstructured flakes having a diffraction grating or a symbol are often need additional layers e.g. for providing a color-shifting effect. A conventional approach is to use roll-to roll coating. A roll of a sheet of polymer substrate material (also known as a “web”) is passed through a deposition zone and coated with one or more thin film layers. Multiple passes of the roll of polymer substrate back and forth through the deposition zone(s) may be made. The deposited coating is then separated from the polymer substrate and processed into flake. However, mass production of such pigments requires very long deposition substrates, and the roll-to-roll technology is inconvenient in this situation.
Accordingly, it is desirable to provide a method of manufacturing microstructured multilayered pigment flakes that overcome the limitations of the techniques discussed above.
All-dielectric interference structures may be formed of dielectric layers with different indices of refraction. Various combinations of these layers can be utilized to achieve the desired optically variable effect. The all-dielectric pigment flakes may be microstructured, they may include indicia for security purposes or have a diffraction grating which provides an optically variable effect.
Another object of the present invention is to provide an efficient method for manufacturing all-dielectric microstructured flakes.